1. Field of the Invention
The present invention relates to communication systems, and in particular to capacity management in communication systems.
2. Description of the Related Art
In communication systems, the functionality of the system is generally defined in the form of protocols, formal statements of the procedures that are adopted to ensure communication between two or more functions within the same layer of a hierarchy of functions. This means that the devices must follow the defined protocol to implement a connection between them. The protocol can be based on a proprietary statement and thus be a result of agreements and decisions made during the creation process of the product or a release of the product. The protocol can also be agreed in co-operation with representatives of the different fields in the industry. During the recent years, standardisation and de facto standards have, due to the customer benefit introduced therewith, made a huge impact on the communication market.
While the released system typically provides an optimised solution to the needs and problems identified and considered during its creation, it is evident that one solution cannot be optimally suitable for all purposes as such. In capacity considerations, the problem typically originates from the fact that the traffic profiles created by the special modes of operation in the realised network differ quite far from the normative ones that have been used as a basis for standardisation. Therefore, the real traffic, when processed according to the pre-defined protocols, will correspondingly accumulate unevenly into the specified channels of the system, and potentially one or more of the channels will get congested. Although in modern solutions the possibility to increase and decrease the number of system elements allows a lot of latitude to capacity planning, dimensioning the whole system according to one, essentially dominating parameter does not necessarily lead to an optimised network configuration, especially when the related costs are considered.
Taking an example from telecommunication, in the early stages of the GSM (Global System for Mobile Communications) breakthrough, it became evident that the critical needs of the mobile professional users could not be met with this mainstream technology. Appreciating this, and considering the predicted potential market of public safety and professional cellular users, TETRA (TErrestial Trunked RAdio), an open digital Trunked radio standard, was defined under the auspices of ETSI (European Telecommunications Standards Institute). As the number of TETRA implementations in the public safety sector is growing, the interest to benefit from the valuable combination of advanced services and reasonable costs provided by TETRA is correspondingly increasing in the other professional user segments as well.
Examples of the above are the public transportation companies, for which TETRA has been considered as a favourable technology. This is largely due to the fact that they need to provide advanced communication to complex fleets typically within an extremely controlled budget framework. Due to the communication environment onboard the vehicles, voice communication in these networks is tightly regulated and mainly dominated by downlink group communication. On the other hand, due to extensive use of vehicle location systems, there exists a need to deliver small amounts of location data frequently from the vehicle to the dispatching system. Due to the specific size of the location data packets, and the established availability of service during voice communication, TETRA Short Data Service (SDS) is considered an optimal bearer for this purpose.
The SDS messages are delivered in signalling channels, which are shared by all mobile stations of the system. Because these signalling channels, especially the Main Control Channel (MCCH), also carry messages related to, for example, random access and call setup, the temporary load of MCCH has a big effect on the success of these functions, and thus additional load incurred by e.g. the delivery of location data needs to be carefully considered. Extensive use of vehicle tracking easily leads to a situation where, in order to provide sufficient MCCH capacity, the number of sites would have to be considerably increased, even though there was no need for additional traffic channel capacity. All related costs considered, such an arrangement is far from optimal.
Generically the problem of load accumulation to at least one channel by one service or by two or more mutually competing services can inherently be detected in any communication system where mapping of the services is pre-defined, by standard specifications of a standardization body or by de facto standards, or internally during product creation process (proprietary systems), to a limited number of channels. As for TETRA, this shows, when used for public transportation communication system, as loads incurred to the main signalling channel by essentially simultaneous delivery of e.g. location data, random access and call setup signalling.
A mechanism introduced in the prior art is the increase of the critical capacity by increasing the amount of the channels that will potentially get congested. For example, in the TETRA standard this has been brought up as a definition of secondary control channels. For each TETRA base station there is one MCCH, in one of the slots of the main carrier of the base station. To increase the signalling capacity, one or more additional slots of the main carrier can be assigned as Secondary Control Channels (SCCH). However, though the signalling capacity is hereby increased, definitions related to the use of secondary control channels are quite rigid and do not accommodate dynamic changes that, on the other hand, are typical of most of the use cases where additional MCCH capacity is needed. A solution, where dynamic changes, e.g. in the operations or organisations of the users, would incur mandatory visits to all the sites in the network, is not viable as such for this purpose. It should be noted that the installed base of TETRA systems is already considerable so any changes to the air interface are very unlikely. The preferred solution should not essentially affect the terminal interoperability of the existing networks.